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The Lhasa Terrane: Record of a Microcontinent and Its Histories of Drift and Growth

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The Lhasa Terrane: Record of a Microcontinent and Its Histories of Drift and Growth Earth and Planetary Science Letters 301 (2011) 241–255 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl The Lhasa Terrane: Record of a microcontinent and its histories of drift and growth Di-Cheng Zhu a,⁎, Zhi-Dan Zhao a, Yaoling Niu a,b,c, Xuan-Xue Mo a, Sun-Lin Chung d, Zeng-Qian Hou e, Li-Quan Wang f, Fu-Yuan Wu g a State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China b Department of Earth Sciences, Durham University, Durham DH1 3LE, UK c School of Earth Sciences, Lanzhou University, Lanzhou 730000, China d Department of Geosciences, National Taiwan University, Taipei 106, China e Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China f Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China g Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China article info abstract Article history: The Lhasa Terrane in southern Tibet has long been accepted as the last geological block accreted to Eurasia Received 15 May 2010 before its collision with the northward drifting Indian continent in the Cenozoic, but its lithospheric Received in revised form 30 October 2010 architecture, drift and growth histories and the nature of its northern suture with Eurasia via the Qiangtang Accepted 2 November 2010 Terrane remain enigmatic. Using zircon in situ U–Pb and Lu–Hf isotopic and bulk-rock geochemical data of Available online 26 November 2010 Mesozoic–Early Tertiary magmatic rocks sampled along four north–south traverses across the Lhasa Terrane, Editor: T.M. Harrison we show that the Lhasa Terrane has ancient basement rocks of Proterozoic and Archean ages (up to 2870 Ma) in its centre with younger and juvenile crust (Phanerozoic) accreted towards its both northern and southern Keywords: edges. This finding proves that the central Lhasa subterrane was once a microcontinent. This continent has zircon U–Pb dating and Hf isotope survived from its long journey across the Paleo-Tethyan Ocean basins and has grown at the edges through Mesozoic–early Tertiary magmatic rocks magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and lithospheric architecture subsequent collisions with the Qiangtang Terrane to the north and with the Indian continent to the south. tectonomagmatic evolution Zircon Hf isotope data indicate significant mantle source contributions to the generation of these granitoid Lhasa Terrane rocks (e.g., ~50–90%, 0–70%, and 30–100% to the Mesozoic magmatism in the southern, central, and northern Tibet Lhasa subterranes, respectively). We suggest that much of the Mesozoic magmatism in the Lhasa Terrane may be associated with the southward Bangong–Nujiang Tethyan seafloor subduction beneath the Lhasa Terrane, which likely began in the Middle Permian (or earlier) and ceased in the late Early Cretaceous, and that the significant changes of zircon εHf(t) at ~113 and ~52 Ma record tectonomagmatic activities as a result of slab break-off and related mantle melting events following the Qiangtang–Lhasa amalgamation and India–Lhasa amalgamation, respectively. These results manifest the efficacy of zircons as a chronometer (U–Pb dating) and a geochemical tracer (Hf isotopes) in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics. © 2010 Elsevier B.V. All rights reserved. 1. Introduction west) (cf. Audley-Charles, 1983, 1984, 1988; Dewey et al., 1988; Kapp et al., 2007; Matte et al., 1996; Metcalfe, 2010; Sengör, 1987; Yin and The Tibetan Plateau is a geological amalgamation of several Harrison, 2000; Zhang et al., 2004), and as having an Andean-type continental collision events since the Early Paleozoic (cf. Dewey active continental margin in the south prior to the collision with the et al., 1988; Kapp et al., 2007; Yin and Harrison, 2000; Zhu et al., northward moving Indian continent in the Cenozoic marked by the 2009a, 2010). The Lhasa Terrane is the southernmost Eurasian block Indus–Yarlung–Zangbo suture (IYZSZ; Fig. 1a) (e.g., Aitchison et al., speculated as having rifted from Gondwana in the Triassic or the mid- 2007; Mo et al., 2008; Rowley, 1996; Yin and Harrison, 2000). to late Jurassic and drifted northward across the Tethyan Ocean basins However, the lithospheric architecture of the Lhasa Terrane (e.g., the before it collided with Eurasia along the Bangong–Nujiang suture age, composition and spatial distribution of crustal lithologies) and (BNSZ; Fig. 1a) in the Cretaceous (earlier in the east and later in the the nature of its northern suture (BNSZ) with Eurasia via the Qiangtang Terrane remain poorly understood. Geological studies in this regard have been hampered by complex crustal deformation as a ⁎ Corresponding author. State Key Laboratory of Geological Processes and Mineral result of continued convergence of the Indian lithosphere against the Resources, China University of Geosciences, 29# Xue-Yuan Road, Haidian District, Beijing 100083, China. Tel.: +86 10 8232 2094; fax: +86 10 8232 2094. Lhasa Terrene through underthrusting (e.g., Kosarev et al., 1999). For E-mail address: [email protected] (D.-C. Zhu). example, despite much research on the Mesozoic geology of the Lhasa 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.11.005 242 D.-C. Zhu et al. / Earth and Planetary Science Letters 301 (2011) 241–255 Fig. 1. Tectonic framework of the Tibetan Plateau and the Lhasa Terrane. (a) Showing the Lhasa Terrane in the context of the Tibetan Plateau. (b) The geology of the Lhasa Terrane where four north–south sampling traverses of Mesozoic–early Tertiary magmatic rocks are indicated by sample locations aided by shaded bands with arrows. The ovals with numerals give crystallization ages in Ma using in situ zircon U–Pb dating techniques (see supplementary online data for analytical details). Abbreviations: JSSZ=Jinsha suture zone; BNSZ=Bangong–Nujiang suture zone; SNMZ=Shiquan River–Nam Tso Mélange Zone; LMF=Luobadui–Milashan Fault; IYZSZ=Indus–Yarlung Zangbo Suture Zone. SL=southern Lhasa subterrane, CL=central Lhasa subterrane, NL=northern Lhasa subterrane. Terrane, the basement rocks are only found in the Amdo and SW 2. Mesozoic–Early Tertiary magmatism in the Lhasa Terrane and Nyainqêntanglha Ranges (Dewey et al., 1988; Guynn et al., 2006; Hu samples et al., 2005; Xu et al., 1985), and the origin of the extensive Mesozoic magmatism continues in dispute (Chiu et al., 2009; Chu et al., 2006; The Lhasa Terrane is one of the four huge W–E trending tectonic Coulon et al., 1986; Harris et al., 1990; Ji et al., 2009a; Kapp et al., 2005, belts (i.e., the Songpan–Ganzi belt, Qiangtang, Lhasa and the 2007; Pearce and Mei, 1988; Zhang et al., 2010a; Zhu et al., 2009b, c). Himalaya) of the Tibetan Plateau (Fig. 1a). According to different Seismic tomography is useful, but it can only tell us the present-day sedimentary cover rocks, it can be divided into northern, central, and snapshot of the lithosphere architecture (Kosarev et al., 1999; southern subterranes, separated by the Shiquan River–Nam Tso McKenzie and Priestley, 2008; Royden et al., 2008), providing no Mélange Zone (SNMZ) and Luobadui–Milashan Fault (LMF), respec- information on material histories. tively (Figs. 1a–b). Apart from the widespread Cretaceous–early The recent analytical advances that combine in situ U–Pb dating Tertiary Gangdese batholiths and Linzizong volcanic succession that and Hf-isotope analysis on zircons (Kemp et al., 2006; Scherer et al., have been known for decades in the southern Lhasa subterrane 2007) from magmatic rocks make it possible to unravel the nature, (Coulon et al., 1986; Harris et al., 1990; Ji et al., 2009a; Lee et al., 2009; history, and lithosphere architecture of the Lhasa Terrane. Zircons, Mo et al., 2007, 2008; Pearce and Mei, 1988; Wen et al., 2008), which are abundant in the more felsic magmatic rocks, can provide abundant Mesozoic magmatic rocks (though poorly dated) are also precise crystallization ages of the host magmas (i.e., U–Pb dating) and present in the central and northern Lhasa subterranes (Fig. 1b) (Chiu can also tell whether the host magmas result from remelting of the et al., 2009; Chu et al., 2006; Coulon et al., 1986; Guynn et al., 2006; ancient mature crustal materials or involve newly-derived mantle Harris et al., 1990; Zhu et al., 2008a, 2009a). material for net crustal growth (i.e., Hf isotope tracer) (Griffin et al., In the southern Lhasa subterrane, the sedimentary cover is limited, 2002; Kemp et al., 2006; Scherer et al., 2007). Importantly, their mainly of Late Triassic–Cretaceous age (Pan et al., 2006; Zhu et al., physiochemical resistance allows zircons to survive from subsequent 2008b). The known Mesozoic volcanic rocks in this subterrane include geological events. As a result, zircons can record information on their mafic and silicic varieties of the Lower Jurassic Yeba Formation (190– geological histories and have thus become a powerful tracer for 175 Ma) (Zhu et al., 2008b) and adakite-like andesitic rocks of the Upper studying crustal evolution (Kemp et al., 2006; Scherer et al., 2007). Jurassic–Lower Cretaceous Sangri Group (Zhu et al., 2009c)(Fig. 1b). In this paper, we report a combined in situ U–Pb dating and Hf- Recent zircon U–Pb age results indicate that the Mesozoic plutonic rocks isotope analysis on zircons from Mesozoic–early Tertiary magmatic in this subterrane were emplaced over a long time period from the Late rocks sampled along four north–south traverses across the Lhasa Triassic (ca. 205 Ma) to Late Cretaceous (ca. 72 Ma) (cf. Ji et al., 2009b).
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